This invention relates to a process for the preparation of a water-soluble, partially-hydrolyzed, solid acrylamide polymer. Partially-hydrolyzed acrylamide polymers have found wide-spread commercial utility as coagulants for various suspensions. Recently, they are also employed as secondary recovering agents for mining petroleum resources.
An acrylamide polymer is generally obtained by polymerizing acrylamide alone or copolymerizing acrylamide and another monomer copolymerizable with acrylamide in an aqueous medium. A partially-hydrolyzed polymer may also be obtained by treating the thus-prepared acrylamide polymer with an alkaline substance. Such a polymer formed in water or subjected further to hydrolysis is in the form of an extremely-viscous hydrous gel containing a great deal of water. In the industry, such a hydrous gel is dehydrated into a solid polymer. As an industrially-applicable preparation process of a partially-hydrolyzed, solid acrylamide polymer, U.S. Pat. No. 4,146,690 teaches dividing a hydrous gel of a polymer, which has been obtained by polymerization of an aqueous solution of acrylamide, into grains, mixing an aqueous caustic alkali solution with the thus-formed grains and subsequently drying them by hot air. In this process, the hydrolysis reaction of the polymer is allowed to proceed only to the extent of about 20-30% of the intended percentage of hydrolysis during its mixing with the caustic alkali. The remaining part of the hydrolysis reaction is allowed to take place in the hot-air drying step. Namely, the added caustic alkali adheres substantially in its entirety on polymer grains but the hydrolysis reaction of the polymer grains does not take place to any significant extent while the polymer grains are mixed with the caustic alkali, because the reaction velocity of hydrolysis is slow at low temperatures. The hydrolysis is brought to completion in the subsequent hot-air drying step, owing to the action of the caustic alkali adhered on surfaces of the polymer grains.
However, polymers hydrolyzed in accordance with the above-described process tend to contain some components which will not be completely dissolved in water. Use of such polymers, which contain water-insoluble components, as coagulants or secondary petroleum-recovering agents may bring about some undesirable results. Accordingly, it is desirable to minimize such water-insoluble components to the extent possible.
An acrylamide polymer hydrolyzed in accordance with the above-described process is susceptible of forming an insoluble precipitate in a solution containing abundant calcium and sodium ions. U.S. Pat. No. 3,039,529 suggests that the formation of such a precipitate renders the polymer unsuitable for use in secondary oil recovery.
U.S. Pat. No. 3,039,529 has also proposed to employ a polymer having a degree of hydrolysis of 12-67%, and preferably 12-45% in order to avoid the formation of precipitate in a solution containing calcium and sodium ions at high concentrations. As a process for obtaining such a hydrolyzed polymer, it also discloses adding sodium hydroxide to an aqueous solution containing 0.934% of a polymer, carry out its hydrolysis for 5 hours at 90.degree. C., and then pouring the liquid reaction mixture into the same volume of methanol so as to cause the resultant hydrolyzed polymer to precipitate. Use of such a dilute polymer solution is however uneconomical because a great deal of energy or a dehydrating agent such as methanol is indispensable for removing the water and obtaining the resultant polymer in a solid state.
The present inventors have conducted research with a view toward developing a process for preparation of a water-soluble, partially-hydrolyzed, solid polymer which contains less water-insoluble components and forms less precipitate in a solution containing calcium and sodium ions. As a result, it has been found that the contents of water-insoluble components and the formation of precipitate can both be reduced when the hydrolysis of an acrylamide polymer is effected by allowing the acrylamide polymer to contact a caustic alkali under certain specific conditions.